On the hypothesis that the mechanism of chemical carcinogenesis is due to abnormal cellular differentiation, we have chosen to follow the course of chemical carcinogens in their interactions with mammary gland during mammary organ development and differentiation. The choice is based on the logic that study of the route of a carcinogen in the susceptible organ is likely to lead to the identification of the critical molecular target of carcinogens in cancer causation. Accordingly, we will examine the following two investigative approaches concerning the actions of chemical carcinogens on mammary gland: a) Mammary gland differentiation and carcinogenesis. The production of preneoplastic-like lesions and carcinogen-protein adducts by chemical carcinogens throughout organ development and differentiation of mammary gland. b) Enhancement of the actions of carcinogens on mammary gland.